The invention relates generally to inspection systems and more specifically to electromagnetic acoustic transducers for use in ultrasound systems.
Ultrasonic inspection is a commonly used non-destructive evaluation (NDE) technique for detecting corrosion and cracking in different objects such as metallic components, pipes, etc. Electromagnetic acoustic transducers (EMATs) are ultrasonic transducers that couple acoustic energy into and out of an object electromagnetically rather than through the use of an acoustic coupling medium such as water.
An EMAT generates ultrasonic energy in a conductive sample under test as follows. A magnetic field is applied to the sample using a magnet, and radio frequency (RF) eddy-currents are induced into the sample using a RF coil. The RF pulse interacts with the magnetic field to produce a mechanical force, which in turn produces ultrasonic waves at the radio frequency.
The absence of a coupling medium enables EMAT transducers to be used in a variety of applications where the use of the coupling medium between the transducer and the specimen is either impractical or undesirable. For example, if the specimen is coated with an acoustically attenuating material, acoustic wave propagation is not feasible. The absence of the coupling medium also improves reliability of the scanning process and hence reduces the risk of having to perform re-inspections in cases where the coupling medium has been lost. The use of EMATs enables inspection at elevated temperatures, on moving objects, in vacuum or oily or rough surfaces and also in remote locations.
EMATs typically have greatly reduced transduction efficiency compared with acoustically coupled transducers. Thus, they are not generally adaptable for use with existing ultrasonic inspection equipment such as ultrasound (UT) pulser-receivers or flaw detectors. Typically, equipment manufacturers design custom EMAT driver/receiver electronics using high-power tone-burst generators. However, these systems typically generate low-bandwidth signals compared to conventional UT pulser-receivers, which limit inspection resolution.
Most EMATs generally include a magnet and a radio frequency (RF) coil that are arranged in close proximity with each other. Such an arrangement causes parasitic eddy currents to be induced in the magnet thus causing the RF field to cancel out rapidly when the EMAT is lifted off from the object's surface thus reducing the lift-off performance of the probe. The close proximity of the RF coil and the magnet also decreases the transduction efficiency of the transducer.
Thus, there is a need for designing an electromagnetic transducer that is capable of being used with existing ultrasound receiver systems and which also has improved transduction efficiency and lift-off performance.